ABSTRACT Dysregulation of endocytosis and endosomal trafficking pathways contributes to the pathogenesis of Alzheimer?s disease and related dementia (ADRD). Beside controlling amyloid precursor processing and ?-amyloid (A?) production in neuronal cells, endocytosis and endosomal trafficking pathways also exist in brain endothelial cells and govern the A? clearance cross Blood-brain barrier (BBB) through receptor mediated transport (RMT). A properly functioning RMT is highly selective due to the spatial distribution of the receptors and specific interaction with their ligands, which ensures the exclusive entry of essential peptides and proteins into the brain and effective clearance of toxic waste from brain to blood in maintaining CNS health and functions. However, our understanding of this unique RMT system within the BBB remains very limited. RMT is tightly regulated by products of AD risk genes, such as Apolipoprotein E and phosphatidylinositol-binding clathrin assembly protein (PICALM). PICALM, a highly validated risk gene for Alzheimer?s disease, is also an endosomal protein and a key component of the RMT machinery at the BBB. PICALM controls the RMT transcytosis across the BBB by facilitating the clathrin-mediated endocytosis and intracellular trafficking of cell surface receptors and their ligands. PICALM deficiency in mice results in defected transferrin trafficking and diminished brain clearance of Alzheimer?s amyloid-? peptides (A?) across the BBB. Therefore, delineate the molecular mechanism of PICALM-mediated transcytosis events offers new opportunities in advancing our understanding of the BBB RMT system, as well as its role in AD pathogenesis. Through in-depth analysis of PICALM?s interactome and functional target validations using an in vitro BBB model, we found that PICALM interacting mitotic regulator (PIMREG) is a novel functional partner for PICALM in the brain endothelial cells and is required for the later steps of RMT transcytosis. Therefore, we hypothesize that PIMREG is an integral component of the RMT machinery, and teams up with PICALM and other proteins in controlling the intracellular trafficking and transcytosis of cargo vesicles, which is essential for brain homeostasis and A? clearance. To test our hypothesis, we propose to determine the function of PICALM-PIMREG complex in controlling endosomal trafficking events in primary brain endothelial cells (AIM 1), understand the role PIMREG in regulating the PICALM-mediated RMT transcytosis of different ligands across the in vitro BBB model (AIM 2), and probe PIMREG?s function in vivo for PICALM-dependent A? clearance across the BBB using antisense oligonucleotides (ASOs) (AIM 3). We expect to generate unique new insights into the biology and molecular mechanism of RMT transcytosis at the BBB, and provide first-hand evidence of a novel component of PICALM- dependent A? clearance across the BBB, which will expand our understanding of RMT and its roles in BBB dysfunction and consequent neurodegeneration in ADRD.